366 The Theory of Natural Selection 



The theory of sexual selection was developed for this 

 purpose. From Darwin's point of view, sexual selection 

 meant the effect produced by the preference which the 

 female of the species exercised in the choice of her mate. 

 The beautiful plumage or song of the male bird would have 

 a competitive value and the superior males would alone have 

 the opportunity to leave offspring. Among these, in turn, 

 the males would show the desirable qualities in an enhanced 

 degree. This theory assumes an aesthetic discrimination on 

 the part of birds, mammals, insects, etc., which it is difficult 

 to establish. It assumes further that the selection is so severe 

 that only the few superior males would have an opportunity 

 to leave progeny. 



With respect to certain moths, Fabre and others have 

 shown that the difference between the male and the female 

 wing pattern plays no part whatever in reproduction. The 

 male finds the female through the sense of smell and the two 

 sexes are unmindful of the wing patterns, whether in the light 

 or in the dark. Where the wings of the male were attached 

 to the female, or vice versa, the behavior of the animals 

 remained unaffected. 



Wallace interpreted sexual selection in a somewhat dif- 

 ferent way. In the case of the birds at least, he noted that 

 it is the female that usually sits on the eggs and would be 

 exposed to danger if she were too conspicuous in her color- 

 ing, or attracted too much attention through her song. 



It is undoubtedly true that the peacock's tail and the 

 nightingale's song play a part in the courtship of these ani- 

 mals. The male walrus or stag is more of a fighter than the 

 female of the species, and the competition for females will 

 give a stronger or more belligerent male the advantage. In 

 this sense there is a selection since only the more effective 

 males will leave progeny. Generally speaking, however, the 

 facts to be observed among animals of many orders do not 

 support the idea that sex differences have arisen or have 

 developed because of the kind of advantage which is im- 

 plied by the theory of sexual selection. This theory as a 



